1. Field of the Invention
This invention concerns pressure vessels, for example, high pressure gas cylinders.
2. Discussion of Prior Art
Such pressure vessels are currently manufactured in aluminium, steel and composite materials. These vessels need to have excellent fracture and fatigue properties. Repeated cycling of pressure inside the vessel causes the vessel to flex, and flexing encourages propagation of any cracks that may appear at the metal surface. Fatigue crack initiation and growth in such vessels occurs at those points where pressure cycling causes maximum flexing (change in strain). This invention concerns treatment of pressure vessels to improve their resistance to fatigue and prevention of premature burst failure.
An established method for improving the fatigue resistance of tubes and cylinders is known as autofrettage. This involves applying a pressure within the bore of the cylinder or tube sufficient to plastically deform the metal at the inner surface. The technique produces compressive residual stresses near the bore, and thus enhances the fatigue resistance of the tube or cylinder subjected to cyclic internal pressure loading. The technique has been applied to continuous lengths of thick walled tubing for at least 70 years.
Autofrettage has also been applied to pressure vessels known as full wrap cylinders, whereby generally a complete thin-walled metal e.g. aluminium inner liner is put into compression. This invention is not concerned with full wrap cylinders of that kind.
U.S. Pat. No. 3,438,113 describes the application of autofrettage to metallic pressure vessels, with the object of increasing the permissible internal pressure loading of the vessel. The invention involves performing autofrettage with the vessel at elevated temperature.
Fatigue failure of pressure vessels such as high pressure gas cylinders particularly those with flat bottoms normally occurs, not in the cylindrical wall, but at or adjacent the closed end of the vessel. This invention arises from the idea that the autofrettage technique might be used to improve the fatigue performance of such closed-end vessels.
In one aspect the invention provides a method of treating a pressure vessel of aluminium or an Al alloy, having a cylindrical side wall and a closed end and having, when at service pressure, at least one region of peak stress located at an internal or external surface of or adjacent the said closed end,
which method comprises subjecting the inside of the vessel to autofrettage by applying a pressure sufficient to plastically deform the said at least one region, said plastic deformation being confined to less than 25% of the wall thickness,
whereby the treated pressure vessel has the property that, when at elevated pressure, each region of peak stress is located away from any internal or external surface at a distance less than 25% of the wall thickness from said internal or external surface.
A region of peak stress is defined as one where the local stress decreases in all directions with increasing distance from the region.
An effect of this treatment is to reduce the absolute value of the peak stress (when the cylinder is under any pressure above atmospheric and less than the autofrettage pressure) in the region of stress raisers (discussed below), and to move the position of peak stress away from a surface of the vessel. Thus in another aspect the invention provides a pressure vessel of aluminium or an Al alloy having an axis, a cylindrical side wall and a closed end joined to the side wall at a knuckle, and having the property that, when at elevated pressure, a region of peak stress is located, within the material of the vessel away from any internal or external surface at a distance less than 25% of the wall thickness from said internal or external surface, in the knuckle and/or axially of the vessel in the closed end. Preferably the said region of peak stress is located within the material of the vessel at least 0.5 mm away from any internal or external surface.
Surface flaws are tears, pits, creases and are typically up to 1-200 xcexcm deep. If regions of peak stress coincide with these surface flaws, they tend to propagate. Moving regions of peak stress at leas 0.55 mm into the interior of the material of the vessel should reduce or avoid this problem.
Autofrettage is normally performed at ambient temperature. At temperatures substantially above ambient, the creep properties of aluminium become more pronounced, and this progressively reduces the beneficial effects of autofrettage.
The vessel may be of any aluminium (including alloys where aluminium is the major component) material that can be formed into an appropriate shape and provide sufficient properties such as mechanical strength, toughness and fatigue and corrosion resistance. Among aluminium alloys, those of the 2000, 5000, 6000 and 7000 Series have been used to make pressure vessels and are preferred for this invention. The vessel is preferably formed by extrusion.
Although hot extrusion according to the invention is possible, cold or warm extrusion is preferred as being a lower cost procedure. Cold or warm extrusion may also give rise to an extrudate having a better combination of strength and toughness properties. The preferred technique is backward extrusion. This technique involves the use of a recess, generally cylindrical, with parallel side walls, and a ram to enter the recess, dimensioned to leave a gap between itself and the side walls equal to the desired thickness of theextrudate. An extrusion billet is positioned in the recess. The ram is driven into the billet and effects extrusion of the desired hollow body in a backwards direction. The forward motion of the ram stops at a distance from the bottom of the recess equal to the desired thickness of the base of the extruded hollow body. Extrusion speed, the speed with which the extrudate exits from the recess, is not critical but is typically in the range 50-500 cm/min. Lubrication can substantially reduce the extrusion pressure required.
The initial extrudate is cup-shaped, with a base, parallel side walls and an open top. The top is squared off and heated, typically induction heated to 350-450xc2x0 C., prior to the formation of a neck by swaging or spinning. The resulting hollow bodyis solution heat treated, quenched, generally into col water, and finally aged.
The requirements of backward extrusion place constraints on the shape of the closed end of the resulting vessel, particularly the base and a knuckle by which the base is joined to the cylindrical side wall. Other production techniques may place other constraints on the geometry of the vessel.
The inventors have performed finite element analysis which shows tht the major stress raisers in such hollow bodies are located in two places: on the inside of the vessel at the knuckle where the base joins the side wall; and on the outside of the vessel at the centre of the base. The relative values of these stress raisers may depend on the cylinder wall and base thicknesses, the dimensions particularly the diameter of the vessel, and the particular base geometry chosen, especially the internal base radius of the knuckle. The method of the invention involves applying a pressure within the vessel sufficient to cause plastic deformation of the metal at one or both of these regions. The applied pressure must obviously not be so great as to burst the vessel, and is preferably less than that required to cause plastic deformation of metal throughout the thickness of the base or knuckle. The applied pressure may be such as not significantly to plastically deform the side wall of the vessel. Alternatively, any plastic deformation of metal in the side wall should be confined to a region at or adjacent the inner surface thereof, e.g. less than 25% and preferably less than 10% of the wall thickness.
The effectiveness of autofrettage in improving fatigue performance does depend on the design of the closed end of the pressure vessel. Thus for example pressure vessels with hemispherical closed ends do not have regions of peak stress and do not show the advantages of autofrettage described herein. More usually, the closed ends of pressure vessels will have semi-ellipsoidal or torispherical dish shapes, and the fatigue resistance of these can generally be improved by autofrettage as described herein. For further description of these shaped ends, reference is directed to an ASME boiler and Pressure Vessel Publication Code 1, Section VIII, Divisions 1 and 2. The effect of end shape is further described in Example 7 below. As there explained, positive advantages do result from designing a pressure vessel with a closed end joined to a cylindrical side wall by a knuckle whose fatigue properties can be improved by autofrettage.
Aluminium high pressure gas cylinders are usually designed so that the stress in the cylindrical side wall at service pressure does not exceed half the alloy yield stress, and that the cylinder burst pressure is at least 2.25 times the operating pressure. In a 7000 Series alloy cylinder having for example a yield stress of 450 MPa, the design should be such that wall stresses do not exceed 225 MPa. Bearing in mind the required burst pressure, it is possible to calculate the degree of over-pressurisation needed for the internal surface of the cylindrical side wall to start to yield. (Wall stresses at the service pressure are higher at the internal surface unless an autofrettage effect is involved). Calculations for a 175 mm diameter 7000 series alloy cylinder having yield atrength of 450 MPa and a wall thickness of 7.9 mm show that pressurisation to at least 85% and often more than 95% of the burst pressure is needed before the stresses in the cylinder side walls exceed the yield stress. Thus treatment of these cylinders by autofrettage is possible under conditions which do not cause plastic deformation in the side wall. Indeed such treatment is advantageous, for autofrettage at pressures close to the actual burst pressure may lead to problems in manufacture owing to variability in material properties, which for example may lead to unwanted permanent expansion ofthe cylinder (BS 5045: Part 3: 1984, Section 20.4, Volumetric Expansion Test) and therefore would not be recommended as a commercial practice.
The autofrettage pressure is likely to be from 75 to 95%, e.g. 75 to 90%, of the burst pressure of the vessel. A finite element analysis of the effects of the over-pressurisation can be performed to show that the right sort of residual stresses are obtained.
Finite element analysis (FEA) is a useful and powerful technique for determining stresses and strains in structures or components too complex to analyse by strictly analytical methods. With this technique, the structure or component is broken down into many small pieces (finite number of elements) of various types, sizes and shapes. The elements are assumed to have a simplified pattern of deformation (linear or quadratic etc.) and are connected at xe2x80x9cnodesxe2x80x9d normally located at corners or edges of the elements. The elements are then assembled mathematically using basic rules of structural mechanics, i.e. equilibrium of forces and continuity of displacements, resulting in a large system of simultaneous equations. By solving this large simultaneous equation system with the help of a computer, the deformed shape of the structure or component under load may be obtained. Based on that, stresses and strains may be calculated (See xe2x80x9cThe Finite Element Methodxe2x80x9d, 3rd Edition, the third expanded and revised section of xe2x80x9cThe finite element method in Engineering Sciencexe2x80x9d, O. C. Zienkiewicz, McGraw Hill Book Company (UK) Ltd, 1977). 
The results of such finite element analysis are shown in FIGS. 1 and 2 of the accompanying drawings, each of which is a von Mises Stress Plot of the lower part of the cylindrical side wall, the knuckle and half the base of an aluminium high pressure gas cylinder repressurised to 24.1 MPa. These were generated using a commercially available ANSYS computer programme, versions 5.0 or 5.1.
These FIGS. 1 and 2 show part of a 175 mm diameter cylinder having a particular base profile, a burst pressure of 49.7 to 51.8 MPa and an assumed working pressure of 24.13 MPa (i.e. 1.17 times the normal design service pressure). The von Mises plot of the residual stress is a useful guide to the stress distribution. In each figure, contour lines within the wall and base of the pressure vessel are lines of equal stress value, the values of which are indicated by the letters A to I.
Referring to FIG. 1, the highest von Mises stress components are shown at the inner surface of the internal knuckle radius (371 MPa) and at the external surface in the centre of the base (377 MPa).
FIG. 2 shows the position again at the assumed working pressure of 24.13 MPa but after autofrettage at 44.82 MPa (i.e. 90% of the theoretical burst pressure). The peak von Mises stress at the knuckle has been reduced to 145 MPa and is positioned a few mm away from the internal surface. The peak stress at the centre of the base has been reduced to a value below 282 MPa and is now positioned several mm from the external surface. In both cases, the depth of the peak stress component is now much greater than the depth of any likely surface flaw. These two effects, the reduction in peak stress and its location change should lead to significant increases in the number of loading cycles needed to initiate fatigue crack from a surface flaw.
These computer predictions are borne out in practice, as demonstrated in the examples below.
Any point in a gas cylinder is in a complex stress state, that is, each point is stressed in more than one direction, such as stresses in the hoop direction, in the radial direction and in the longitudinal direction.
Description of Stress at a Point and Principal Stresses:
In solid mechanics, it is convenient to describe stress at a point within a component or a structure on an infinitesimal cube which centres on the point and whose faces are normal to the axes of a chosen coordinate system. The stress is resolved into three normal stresses and six shear stresses acting on the faces of the cube. Since the choice of the coordinate system and its orientation is somewhat arbitrary or for the convenience of analysis, the levels of the normal and shear stresses may vary with the orientation of the coordinate system. There exists a special orientation of the coordinating system. On the faces of the infinitesimal cube aligned to this particular coordinate system, there are only resolved normal stresses and no resolved shear stresses. These special resolved normal stresses are called principal stresses ("sgr"1, "sgr"2, "sgr"3). The maximum principal stress ("sgr"1) is the greatest of the three and the minimum principal stress ("sgr"3) the least.
von Mises Stress: The mechanical properties (modulus of elasticity, yield stress, work hardening and plastic deformation beyond yielding, etc.) of a ductile material such as an aluminium alloy are normally established through tensile tests. Tensile tests are carried out under uniaxial stress conditions. Stress-strain curves are obtained. In order to conduct stress analysis on a multi-axially stressed component or structure, it is necessary to establish a correlation between the multi-axial stress-strain relationship and the uniaxial stress-strain relationship, especially in the situation of material yielding where Hooke""s law is no longer applicable. von Mises proposed a yield criterion which as been generally accepted as the most suitable for the ductile materials.
Beyond yield, the von Mises stress and equivalent strain (defined in a similar form to von Mises stress) will follow the tensile stress-strain curve. Therefore, von Mises stress may be generally used to assess the severity of the stress state at any point of a component or structure, except when the component or structure is predominantly under hydrostatic tension. A gas cylinder is not under such stress condition.